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Comparing Flow Variability in Urban Streams Across Environmental Settings

Elise M. Giddings1, Dan L. Calhoun2, and Lori A. Sprague3

1 U.S. Geological Survey, 3916 Sunset Ridge Rd., Raleigh, North Carolina 27607
2 U.S. Geological Survey, 3039 Amwiler Rd., Atlanta, Georgia 30360
3 U.S. Geological Survey, Box 25046, Denver Federal Center MS415, Denver, Colorado 80225

Abstract

Landscape transformation during urbanization affects streamflow variability which in turn affects instream biological communities. Climatic and topographic factors influence flow variability as watersheds become more urbanized. As part of the National Water-Quality Assessment Program, variability in streamflow patterns were assessed at 30 sites in each of 6 study areas across the Nation (near Raleigh, NC; Atlanta, GA; Milwaukee, WI; Dallas-Fort Worth, TX; Denver, CO; and Portland, OR). Submersible pressure transducers were installed to record continuous stream stage, which was converted to continuous cross-sectional area using a surveyed channel cross section. Metrics were calculated to represent magnitude, duration, and frequency of high and low streamflow conditions, and these metrics were compared with measures of urban intensity in the landscape. In all six study areas, as urbanization increased, the frequency of quickly rising streamflows increased (Spearman’s rho 0.62 to 0.91), illustrating an increase in stream flashiness. The duration of high streamflow showed variable responses in the study areas. In study areas in the southern U.S. (Atlanta, Raleigh, Dallas-Ft. Worth) the median duration of high streamflow events and the duration of the longest high streamflow event showed a factor-ceiling effect, where streams high in urbanization had short duration events, while those low in urbanization had a range of event length. In the Denver study area, the median and maximum duration of high streamflow showed the opposite effect; the high urban sites had longer duration events, although the range of event lengths was much smaller than other study areas. The Portland and Milwaukee areas showed mixed results. Although each study area had significant hydrologic responses to urbanization, the strongest responses were seen in Atlanta, Raleigh, and Dallas-Ft. Worth. These areas lack confounding effects such as snow cover, hydrologic alterations or agricultural effects.

Giddings, E.M., Calhoun, D.L., and Sprague, L.A., 2006, Comparing flow variability in urban streams across environmental settings: Proceedings of the National Water-Quality Monitoring Council National Monitoring Conference, May 8-12, 2006, San Jose CA.